The present invention relates to an apparatus for measuring distances and, is concerned more particularly with ranging apparatus for controlling the focus of an optical instrument.
It is well known in the art to employ ultrasonic ranging equipment for measuring focal distance. U.S. Pat. Nos. 4,199,246, 4,148,574 and 4,167,316 all disclose sonic ranging systems for determining and setting the focus of cameras.
In the camera field, focus control is necessitated in order to bring the object being photographed into the field of view of the camera lens system. In such instances, the depth of view field, that is the portion of the field which is in focus, may be relatively large, and for this reason, precise control over the focal adjustment is not critical. In certain types of instruments such as photoplotters or laser cutters, the depth of field is limited. This condition of limited field is common with optical systems that focus on objects in close proximity to the lens systems. With high accuracy photoplotters that are used to generate "masters" for printed circuitboards, the photoplate or film being exposed is held very close to the photohead for several reasons, and a depth of field in the range of 0.01 inch is not uncommon. Consequently, the problem of maintaining a properly focused image is acute. Sharp focusing is essential in order to clearly delineate the conductive and nonconductive portions of the circuitboard. Additionally, microcircuits are becoming increasingly more important, and such circuits employ extremely fine conductors in high density arrays. Focus control in the production of such circuits takes on even greater significance.
It is known that glass film plates from which masters are manufactured have surface irregularities. A 20".times.24" sheet of ultra flat film has surface errors, referred to as diagonal curvature, in the order of 0.0019 inch. The overall flatness of a 20".times.24" plate may vary by as much as 0.0112 inch across the photosensitive surface. With high precision photoplotters, these variations in the surface of the glass have a significant impact on the focus of any image projected onto the photosensitive surface in view of the limited depth of field.
Additional factors unrelated to the film plate itself and which also effect focus are found in the structure which supports the plate and produces relative movement between the plate and the optical head. For example, a lack of parallelism between the ways on which the head and plate are moved relative to one another causes the distance between the head and photosensitive surface to vary at different locations on the plate. Also, irregularities in the surface of the ways or on the guides which travel on the ways superimpose additional positioning errors on any lack of parallelism.
For this reason, some prior art photoplotters have been provided with control mechanism to adjust the optical head of the plotter and thereby maintain a substantially uniform distance between the photohead and the photosensitive surface being exposed by the head. U.S. Pat. No. 3,511,149 to Blattner and U.S. Pat. No. 3,704,657 to Sliwkowski are two examples of photoplotters having devices for adjusting the optical head relative to the photographic surface for focus control. In each of these prior art plotters, a pneumatic sensor mounted adjacent the optical axis of the head detects the distance between the head and the photosensitive surface of a film and makes appropriate adjustments in the positioning of the head relative to the surface to hold the focus of the head.
For the reasons mentioned above and the requirements for more accurate control of focus on the film plane, it is a general object of the present invention to provide a ranging apparatus that permits precise automatic focus control in a photoplotter or other optical instrument.